package com.codeforces.gym.g10186;

import java.util.Scanner;

public class B {

	static Scanner scanner = new Scanner(System.in);
	static String str;
	static int count[] = new int[30];
	static int nerase = 0;

	public static void main(String[] args) {
		str = scanner.next();
		for (int i=0;i<str.length();++i) {
			char c = str.charAt(i);
			int val = c - 'a';
			count[val]++;
		}
		int nodd = 0;
		for (int i=0;i<26;++i) {
			if (count[i] % 2 == 1) {
				nodd++;
			}
		}
		if (nodd > 1) {
			nerase = nodd - 1;
		}
		System.out.println(nerase);
	}

}
